Abstract - A debate of directional antennas and performance, the limits or disadvantages and benefits of using directional antennas compare to omnidirectional antennas.
INTRODUCTION
The meaning for directional antennas is antennas which radiates in a single or more directions that allowing to escalates the performance on transmit, receive and decrease the interference from other resources, or in another way to say is the antennas that receives or sends indicators most effectively in a particular. The Radio Regularity (RF) energy can be diverted in a specific direction to father distance. Therefore, it offers long range coverage but the effective beam width lowers.
Due to how big is directional antennas, the frequencies used are above 200 to 300 MHz. The antenna's wideband property will depend on the kind of the antennas and the directional properties of the antennas are a function with their electric powered size.
Figure 1 : Directional Antenna with 6 zones. [6]
Each area is a wedge with radius r spanning /3 radians. Area 1 always faces east. The dashed group shows the omnidirectional communication radius.
A few types of directional antennas can be found. [5]
Fixed Beam
Fixed beam antenna has fixed gain account with a primary lobe pointing within a path. We only can steering the beam by changing the orientation of the antenna in physical form, which permit relatively sluggish changes at best. This kind of antenna does indeed provide high gain for his or her cost and are broadly deployed used.
Figure 2 : Fixed Beam antennas gain pattern
Sector
In sector antennas, there is certainly multiple resolved beams antennas where each of these beam antennas aspires in different route. Each has covering different area and total 360 coverage. Packets may be sent on any sector. Switching between antennas is performed electronically and allows the decision of sector to occur on a per packet basis.
Figure 3 : Sector Antenna Gain Pattern
Analog Phase Array
Analog Stage Array antennas work by calculates phase shifts in to the transmission at antenna's elements. There will be individual impulses after from stage shifts interfere constructively and destructively with each other in order to create a specific gain style.
Figure 4 : Analog Phase Array Antenna Gain Pattern
Digital Period Array
Another explanation for Digital Phase Array makes sense antenna uses digital sign to accomplish stage shifting. Even the excess power required to do this had boost the cost and difficulty greater than the Analog Phase Array, but you can find some significant function had been added. Lobes and nulls may be steered very specifically to amplify desired indicators and eliminate extraneous ones and position of entrance information for packets may be obtained as well. Multiple habits may be became aware simultaneously using the same set of elements.
Figure 5 : Digital Phase Antenna Gain Pattern
THE GAIN OF DIRECTIONAL ANTENNAS [1]
The description for directivity corresponding to [7], "The directivity of a wireless antenna is distributed by the ratio of maximum rays intensity (electric power per unit sound viewpoint) to the common radiation strength (averaged over the sphere). The directivity of any source, apart from isotropic, is obviously higher than unity". Both omnidirectional antennas and directional antennas have directivity but the difference between these two antennas is the coverage structure. For omnidirectional antennas, the coverage structure is torus-shaped. The directivity in directional antennas case in greater than the omnidirectional due to its ability to target the beam.
Because of the greater gain in the directional antennas, compare to onmidirectional, the indication sent with some ability will be able to reach wider distance than the sign sent in the omnidirectional antennas.
LIMITATION S OF DIRECTIONAL ANTENNAS
Deafness [1]
Deafness is one of the problems happen with all the directional antennas and it experienced limited the network performance. Deafness is the situation of didn't hearing from others. In omnidirectional antennas, all neighboring node are capable of hearing all ongoing transmission but not in directional antennas. The node may be turned into particular sector while acquiring and the node reported to be 'locked'. In this situation, all the signals that arrive in other sectors cannot be received by the nodes. The nodes said to be Deaf in every other industries.
Figure 6 : Deafness
In the amount 2 above, it shows that Node A is connecting with Node B. Through the communication, Node A is facing to Node B and switched from Node C. When Node C is sending a transmitting to node A, Node A didn't hear for the transmission unless it can be found. Node A can be said "Deaf" towards Node C.
The put into action of directional antennas thought to reduce the disturbance but it offers increase the percentage of packet reduction. Multiple retransmissions could also cause the node to misunderstanding that the connection is lost credited to freedom and triggering for path learning about search. In another way to state, it triggers the destructive interactions with the top layers.
Drawbacks Specific to Directional Mac pc (DMAC)
The above coating in OSI model will not seem to funnel the top features of the model even there is certainly some or specific changes in the physical coating. Reusing the same methods as that omnidirectional Macintosh personal computer, DMAC has generated or bring new problems which were not is present in omnidirectional Macintosh personal computer. There is a few disadvantages :
Heightened hidden terminal
When a node transmits a sign that may impact an ongoing transmission, the invisible terminal problem will be arise. The Prepared to send and clear to send are not reaching all the neighbor nodes which will cause those nodes unaware of ongoing transmitting.
Head of brand blocking-
The queuing mechanism used is the First-In-First-Out (FIFO). Therefore the node with antennas will select the first packet in the queues to transfer. The node will send the packets if the channel in the direction that the node wish to communicate with. In the event the route is not idle, the node has to wait before route is idle and then transmit the packet. There is still has others packet in the queue holding out to transfer, and there can be done that the channel is not idle. Because of the first packet is still waiting for transmission, it obstructs all the packets that may be transmitted.
Imperfect electronic carrier sensing
Nodes often do not pay attention to all the indicators around them due to the deafness problem. This triggers an incomplete Directional Network Allocation Vector (DNAV) stand which doest not constantly store the condition of the channel in different guidelines. This contributes to imperfect exclusive carrier sensing.
Effect of range of motion [1]
Figure 7 : Effect of mobility
The reach-ability scheduled to raised range (Position 1)
In the communication between two nodes, Node X to Node Y, the coverage using omnidirectional antennas is circle pattern as the coverage routine using directional antennas is within lobes pattern. In the event the Node Y goes from the circle area, it'll unable to obtain any packet transmitted by Node X. Because the gain of directional gain is more higher, it's possible that Node Y continues to be in the directional range X and hence, Node Y still in a position to have the packet sent. If not, Node Y still will be unable to have the packet sent.
Reach-ability in different sector (Position 2)
In any area inside the group area when working with omnidirectional antennas, the Node X will be able to reach the Node Y. While using directional antennas with DMAC, Node X didn't reach the Node Y using the same sector. It is because Node Y has gone out of the selection of the indication that is transmitted for the reason that sector.
Un-reach-ability credited to omni-discovery (Position 3)
Node X attempts to send the packet in its old direction. After failing woefully to reach it after 'Directional retransmit limit', Node Y will tries to send an omnidirectional indication. Because the Node Y is unreachable by directional indication, Node X cannot discover the Node Y and therefore, Node X will assume that the Node Y is unreachable. Node X will never tries to attain the Node Y using different sector though it is be achieved by transmitting directional sign in various sector.
Node X will studies the mistake to the aforementioned routing coating and drops the packet due to no road. In fact this problem can be resolved by transmitting the packet directionally in another sector.
Totally unreachable (Position 4)
This is another case where the Node Y goes to out of range for both and directional. Node X will struggle to reach the Node Y both in omnidirectional and directional. This circumstance of ability to move will lead to disconnection and can't be recovered.
ADVANTAGES OF USING DIRECTIONAL ANTENNAS
Security Issue
Use directional antennas to avoid Wormhole invasion [6]
Figure 8 : Wormhole attack where in fact the adversary handles nodes X and Y and links them by way of a low-latency link.
Wormhole episode means that a forwarded packet from attackers through a high quality out-of-band website link and replays those packet at another locations. The attackers will replay the packet received by node A at node B and vice versa. A far more smart attacker may able to replace the wormhole endpoints at particular locations which may disrupt practically all communications to or from a certain node and also to all the nodes in the communications.
In directional antennas, predicated on the indication received, a node can have the approximate course information. Thus, an attacker cannot execute the wormhole assault if the wormhole transmitter is recognized as a wrong neighbor, that is not the true neighbor and so ignore the announcements. There are three ever more affective protocols to help to avoid the wormhole disorders. As bidirectional information is added, it is more challenging to allot the attacker to establish the wormhole strike efficiently. The three protocols are
Directional neighbor finding process, that is will not rely on any assistance between nodes and cannot prevent many wormhole episodes.
Verified neighbor finding standard protocol, that is stopping wormhole attacks where the attacker control any two endpoints and the sufferer nodes are in least two hops faraway.
Strict neighbor discovery protocol where to prevent wormhole attacks even though the sufferer nodes are near by.
To Support Symmetric traffic services WITH TIME Department Duplex (TDD) Code Department Multiple Access (CDMA) [2]
Code Division Multiple Access consists of two procedures, that are Time Department Duplex and Frequency Department Duplex, which to provide two-way simultaneously. A set of frequency bands is employed for uplink and downlink transmissions in FDD. In TDD, the uplink sand downlink transmissions are multiplexed into time slot machines on a single frequency band, the system is its capability of flexibly to modifying the uplink and downlink bandwidth by allocating different amounts of time slots. It is more suitable for applications with asymmetric traffic suck as Internet Surfing and file copy compare to FDD. However, Cross-slot Interference which may critically degrade the system capacity may happen in TDD-CDMA system during the transmission of asymmetric traffic from adjacent cells. Cross-slot disturbance is the interference due to opposite way transmissions between two adjacent cell.
In the [2], this paper shows that the through the use of developed interference research framework how the interference between virtual skin cells can be suppressed because of the directivity of directional antennas and therefore proposed a digital based disturbance - resolving algorithm to aid asymmetric traffic services in TDD-CDMA Systems.
In this paper, it explained that how the directional antennas take result in assisting the asymmetric system in TDD-CDMA.
By using directional antennas in a trisector mobile system can limit the strong base-base disturbance into a hexagon and therefore, it is possible by just coordinating the transitioning things of downlink and uplink bandwidth ratio in mere three areas for TDD-CDMA.
The cross-slot-interference level in the omnidirectional circumstance is bigger if equate to the directional antennas case. This is because of the transmissions ability from a mobile train station in omnidirectional mobile system is higher than the trisector cellular system. The reason is because the smaller antennas gain.
In random network
Spatial reuse factor
Figure 9 : Spatial reuse in directional antenna
In figure 9, Node A want s to acquire communications with Node B, Node C and Node D. In omnidirectional case, the communication between Node C and Node D is prohibited if there is packet sending from Node A and Node B. That is to avoid that the packet from Node C to hinder Node A to Node B communications. If we using directional antennas, then your sender may concentrate the beam towards to the receiver. It allows that the coomunications between Node A to Node B and Node C to Node D go on currently. As finish, if the nodes use directional antennasthen neighboring nodes that are not in the direction of the sign can go ahead with their transmitting. Multiple transmissions can be initiated by different nodes instead of a single transmitting if they are not interfere with each other. This increasing the spatial reuse factor.
Extended Range and Energy Savings
Figure 10 : Prolonged range in directional antenna
In the figure 10, the Node A want to communicate with Node C. in omnidirectional case, the marketing communications cannot reach in solo hop. Node A has to transfer the packet to Node B and Node B will transfer the packet to Node C. When using directional antennas, you can find bigger directional gain. Hence, Node A can reach the Node C in one hop. With higher directional gain, targeted beam can travel bigger distance than those unfocused beam in omnidirectional beam. The sender can reach to receiver with further away which has increase the transmitting range. Also with higher directional gain, the energy necessary to reach a maximum distance is significantly less than the power used in omnidirectional antennas. This reduce the energy put in by nodes for transmission and reception.
Conclusion
As bottom line, directional antennas have those benefits that is not exist or stronger or solve the situation are present in using omnidirectional antennas but gleam few problem that occur only in using directional antennas. There's a few solution suggested to reduce the problem but there continues to be have a great deal of space for improvement.
